Spread-spectrum modulation has long been used in the transmission of digital signals from different sources over a common channel, its purpose being to insure the privacy of the transmitted messages and to minimize mutual interference among simultaneously transmitted signals. At a transmitting station, outgoing digital signals (which could be analog signals converted into binary form) are multiplied with a random or pseudorandom sequence of binary pulses whose period is a submultiple of the symbol duration; the sequences generated at the transmitting stations are essentially uncorrelated among one another. Such sequences may conform, for example, to the well-known Walsh function. At an associated receiving station, the incoming digital signal distorted by the aforementioned pulse sequence--and possibly encumbered by interfering signals from other users of the same channel--is multiplied with a pulse sequence which is a precise replica of the one used at the transmitting end and is properly synchronized therewith. This procedure re-establishes the original binary signal which is then integrated over limited intervals, equaling the duration of the symbols, to eliminate accompanying interferences. The digital signal thus purged can be reconverted, if desired, into analog form.
Since digital signals require a considerable bandwith for their transmission, spread-spectrum modulation has heretofore been generally limited to military uses. Moreover, the analog/digital conversion at the transmitting end and the digital/analog reconversion at the receiving end are cumbersome procedures which limit the utility of such systems for civilian purposes, e.g. for intercommunication among telephone subscribers.